1. Field of the Invention
This invention relates to low-profile antenna systems and to impedance matching systems and methods for use with low-profile antenna systems. More particularly, this invention relates to novel apparatus and methods for providing a broad band impedance match between a low-profile antenna and a transmission line such that the antenna system may be efficiently operated over a broad range of electromagnetic signal frequencies.
2. The Prior Art
Various types of communication systems which are based upon the propagation of electromagnetic signals have been known and used for many years. For example, commercial radio and television stations broadcast many hours of programming each day by transmitting appropriate electromagnetic signals through the atmosphere which are then received by individual radio and/or television receivers located within such station's area of broadcast. Similarly, by both transmitting and receiving suitable electromagnetic signals, government agencies, private businesses, and individuals are able to readily communicate over long distances, thereby transmitting and receiving data and/or instructions which may be vital to out nation's economy and/or security.
Generally, communication systems such as those described above employ large antennas which extend high above the earth's surface in order to effectively transmit and/or receive the desired electromagnetic signals. Such antennas are commonly referred to as "aerial" antennas. Typical aerial antennas may, for example, be secured several hundred feet above the earth's surface to the top of a high tower or building; and such antennas are also commonly supported by numerous guy wires which provide the antenna with additional structural stability. It is also quite common to install aerial antennas, together with their supporting towers and guy wires, on the slopes of relatively high mountains. By placing the antennas upon such towers and/or mountains, the range and effectiveness of the antennas can be significantly increased.
Although conventional aerial antennas are generally quite effective and may be constructed so as to operate very efficiently in both transmitting and receiving the desired electromagnetic signals, such antennas suffer from a number of significant disadvantages.
First, aerial antennas are considered "soft" for security purposes. "Hardness" and "softness" are military terms used to denote a system's vulnerability to destruction; and the "harder" a system is, the less vulnerable to destruction such system is. The "hardness" of a communication system is generally measured by such criteria as its ability to withstand substantial shock, as in the case of a powerful explosion occurring very near to the system, and the ability of the system to survive high energy electromagnetic pulse radiation which may be produced by a nuclear blast.
Unfortunately, even though a powerful explosion may be centered some distance away from the above-described aerial antennas, the resulting shock waves will likely damage or destroy such antennas, thereby rendering the associated communication systems either totally or partially inoperative. Furthermore, aerial antennas which transmit or receive high frequency electromagnetic signals are very susceptible to the adverse effects of the above-mentioned electromagnetic pulse radiation.
Some attempts have been made to increase the "hardness" of communication systems which use the above-described aerial antennas by constructing appropriate back-up antenna systems. However, both economic and environmental considerations make it very difficult to either justify or construct the number of back-up antenna systems which would be required in order to achieve an acceptable level of "hardness." Therefore, despite the general effectiveness of the conventional aerial antennas, the use of such antennas in communication systems which are vital to our national security remains highly undesirable.
Another significant drawback of prior art aerial antennas is that they generally are quite expensive, cumbersome, and time consuming to construct. Construction of an aerial antenna and its associated support structure may, for example, require several thousand dollars in materials alone. In addition, a number of laborers are usually required in order to complete construction, and conventional construction machinery and equipment are typically employed. Moreover, many man-hours of labor are generally required before the antenna system is operational.
In certain applications, such as, for example, when an antenna system will be used by military personnel, there is simply not enough time to construct a conventional aerial antenna. This is particularly true if the antenna is to be used by combat troops. In such cases, the required construction materials and equipment may also be unavailable, and the man-power requirements may likewise be prohibitive.
In military applications, there are also other important reasons for not using conventional aerial antennas. First, aerial antennas are generally immobile and cannot be easily moved from one location to another. Also, aerial antennas are relatively easy to detect and quite difficult to camouflage. It will be appreciated that both of these factors render conventional aerial antennas unsuitable for many military applications.
As a result of the above-outlined drawbacks of conventional aerial antennas, a number of attempts have been made by those skilled in the art to provide antenna systems which are easily camouflaged, economic to construct, simple to deploy, and resistant to destruction. One type of antenna system which has been investigated and which seems to have great potential may generally be referred to as a "low-profile" antenna system, in that the system is deployed at, or near (i.e., either above or below) the earth's surface.
A low-profile antenna system does not, of course, require the expensive support structure of conventional aerial antennas. As a result, a low-profile antenna system is generally much less expensive to construct than conventional aerial antenna systems. In addition, since a low-profile antenna is positioned on or near the earth's surface, and may, therefore, require little or no supporting structure, the man-power requirements for construction are significantly reduced, and the need for machinery and equipment may be virtually eliminated. Further, since a low-profile antenna system is located on or near the earth's surface, it is much easier to camouflage and is inherently less susceptible to destruction.
Despite the promising possibilities of low-profile antenna systems, however, the low-profile antenna systems of the prior art have generally been unable to provide acceptable transmission and reception characteristics. In particular, the prior art low-profile antenna systems have generally been found to be inefficient in transmission and reception, except over a relatively narrow band of electromagnetic signal frequencies.
While operation of an antenna at a single signal frequency, or over a relatively narrow range of frequencies, may be acceptable for some applications, broad band operation is most desirable. For example, in military applications, messages are generally transmitted at several different signal frequencies, and the signal frequency is often changed in some manner during the transmission. In this way, it becomes much more difficult for unauthorized personnel to intercept the transmitted message, and hostile groups or forces are less likely to be able to jam or distort the transmitted message.
Significantly, since the prior art low-profile antenna systems are generally not suited to broad band operation, such antenna systems are not able to efficiently transmit or receive portions of the message. Only those portions of the message which are transmitted at a frequency which is within the narrow operating band of frequencies for which the low-profile antenna system was designed can be efficiently transmitted or received. Portions of the message which are transmitted at other frequencies may be either weak or lost entirely.
Although some attempts have been made to adapt low-profile antenna systems for operation over a broad range of signal frequencies, such attempts have heretofore proven unacceptable. The prior art attempts to provide a broad band low-profile antenna system have generally been quite cumbersome, requiring complex tuning mechanisms or other system adjustments. As a result, the prior art low-profile antenna systems have not been able to provide acceptable operation characteristics which are needed for many applications.